The invention concerns a dryer section, comprising at least one dryer group that makes use of single-wire draw and/or twin-wire draw and at least one suction roll with which the web to be dried is in direct contact.
As is known from the prior art, in multi-cylinder dryers in paper machines, twin-wire draw and/or single-wire draw is/are employed. In twin-wire draw, the groups of drying cylinders include two wires, which press the web, one from above and the other one from below, against heated cylinder faces. Between the rows of drying cylinders, which are usually horizonal rows, the web has free and unsupported draws, which are susceptible of fluttering, which may cause web breaks, in particular as the web is still relatively moist and, therefore, of low strength. This is why, in recent years, ever increasing use has been made of said single-wire draw, in which, in each group of drying cylinders, there is one drying wire only, on whose support the web runs through the whole group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces, and on the reversing cylinders or rolls between the drying cylinders the web remains at the side of the outside curve. Thus, in single-wire draw, the drying cylinders are placed outside the wire loop, and the reversing cylinders or rolls inside the wire loop.
When paper is dried one-sidedly, the result is a tendency of curling of the sheet. When paper is dried by means of normal groups with single-wire draw from the side of its lower face, and if such asymmetric drying is extended over the entire length of a forward dryer section, the drying takes place so that first the side of the bottom face of the paper web is dried, and, when the drying makes progress, the drying effect also spreads to the side of the top face of the paper web. Thus, the dried paper is, as a rule, curled so that it becomes concave when viewed from above. In view of controlling the tendency of curling of paper, dryer sections have been used in which there are inverted dryer groups.
With respect to the prior art related to the control of curl, reference is made to FI Patent Application 971301, in which a method is described in the dryer section of a paper or board machine for the control of the curl of paper. In the method, steam treatment and/or moistening of the paper web is employed, and the operations carried out in order to control the curl of the paper web are carried out in a number of stages and/or when the temperature of the web is lower than 85xc2x0 C., preferably lower than 75xc2x0 C., and/or when the dry solids content of the web is in a range k1 . . . k2, wherein k1=the ultimate dry solids contentxe2x88x927%, and k2=the ultimate dry solids content+3%. In said patent application, a paper or board machine is described which comprises at least a headbox, a former, a press, and a dryer section, in which steam boxes or moistening devices are employed for the control of the curl. There are at least two curl regulation devices in view of providing a curl regulation taking place in a number of stages.
The paper web can also be dried in dryer sections partly or fully based on impingement drying, and with respect to the prior art related to such dryers, reference is made to FI Patent Application 971713, in which a dryer unit and a dryer section that makes use of the unit are described. In the dryer unit, there is a drying-wire loop, and the dryer unit comprises a large-diameter impingement-drying and/or through-drying cylinder, which has been fitted in the interior of said drying-wire loop, and on the top of which cylinder/in the vicinity of which cylinder, at both sides of the cylinder, smooth-faced heated contact drying cylinders have been fitted, whose diameter is D2 less than D1. The impingement-drying and/or through-drying cylinder has been placed in spaces primarily below the floor level of the paper machine hall and is provided with a blow hood that can be opened and closed, so that removal of broke out of connection with said hood takes place primarily by the force of gravity. The central axes of the heated contact drying cylinders placed in the vicinity of, and at both sides of, the impingement-drying and/or through-drying cylinder have been placed in the vicinity of, or above, the floor level of the paper machine hall, and the wrapping sector b of the paper web to be dried on the outside face of said drying wire over said impingement-drying and/or through-drying cylinder is b greater than 180xc2x0.
With respect to the prior art related to impingement drying, reference is also made to FI Patent Applications 963733 and 951746 (U.S. patent application Ser. No. 08/945224).
In the FI Patent Application 963733, a dryer section of a paper machine is described, which comprises drying modules, in which there is a drying-wire loop guided by guide rolls. The drying modules comprise a large-diameter impingement-drying and/or through-drying cylinder, which has been fitted inside the drying-wire loop. The outer mantle of the impingement-drying and/or through-drying cylinder is provided with grooves and/or it is permeable to the drying gas, and on the sector of contact with the drying wire a drying hood has been fitted, in whose interior, in the vicinity of the outer face of the web to be dried, there is a nozzle field or equivalent, through which a set of drying gas jets can be applied against the free outer face of the web to be dried. In the dryer section, a number of successive drying modules are employed, which have been fitted above the run of the paper web so as to apply drying energy to the paper web primarily from the side of its top face, and of the total drying energy of the dryer section, the major part, about 60 . . . 100%, is applied to the paper web by means of said drying modules from the side of the top face of the paper web to be dried.
In the FI Patent Application 951746, a dryer section concept for a paper/board machine is described, in which the dryer section comprises a number of groups of drying cylinders, which groups are exclusively provided with single-wire draw, on whose support the web is guided, meandering in loop shape, from a suction cylinder onto a drying cylinder and from the drying cylinder onto a second suction cylinder and from it further onto a second drying cylinder and further in the group. At least some of the drying cylinders comprise impingement drying units or equivalent in their connection, through which units a heated medium, preferably air or steam, is passed through the wire into contact with the web to produce a two-sided drying effect and to increase the drying capacity. The impingement drying units have been fitted in the end of the dryer section in such an area of dry solids content of the web that curl of the web can be affected and prevented by means of impingement drying.
With respect to impingement drying, reference is made further to FI Patent 100,013.
With respect to the prior art, reference is also made to FI Patent Application 960925, in which paper machine dryer sections provided with intermediate calendering are described.
Further, reference is made to a prior-art dryer section, in which the suction roll is provided with a drying-wire circulation of its own and which is described in the U.S. Pat. No. 3,816,941.
With respect to the prior art, reference can also be made to the U.S. Pat. No. 3,345,757, in which a dryer section is described in which the web runs around a drying cylinder and there is a blow hood on the cylinder, in connection with which hood an air-conditioning felt guide roll has been fitted.
From the JP publication 5-222692, a solution is known in which heatable suction rolls are used, against which the web is in direct contact. In this cited paper, suction rolls are also described which have a common contact and vacuum area in which a vacuum is applied to the web in the sector that is covered by the paper web. In addition to this, in the paper, impingement drying applied against said suction rolls is described.
With respect to the prior art, reference is also made to FI Patent 87,670, in which a suction roll provided with a resilient mantle is described, which suction roll forms a nip with drying cylinders with single-wire draw. In the arrangement suggested in this cited paper, between the suction roll and the web, there is a drying wire that circulates in the dryer group.
The prior-art arrangements described above involve drawbacks and problems, whose elimination is one of the objects of the present invention.
It is an object of the present invention to provide the dryer section with arrangements by whose means it is possible to obtain an increased drying capacity in the dryer section and, in this way, a more efficient dryer section and, thereby, lower costs.
It is a further object of the invention to provide a dryer section in which it is possible to control the curl of a web.
It is another object of the invention to provide a dryer section at least partly provided with no wire, in which case the cost of manufacture is considerably lower.
A further object of the invention is a dryer section which is connected with pre-calendering and/or in which the regulation of moisture profiles is easy.
Further, it is an object of the invention to provide a dryer section in which cross-direction shrinkage is prevented and/or whose runnability is good and/or in which the removal of broke is easy.
In view of achieving the objectives stated above and those that will come out later, the dryer section in accordance with the present invention is mainly characterized in that the surface construction of the suction roll is of a fine structure so that the web can be passed to run over the suction roll in direct contact with the roll so that the web is not damaged.
In a dryer section in accordance with the present invention, it is possible to use dryer-section suction rolls of at least three different types:
A roll of a normal construction with a drilled mantle and provided with a shrink sock, a roll of which type is in itself known from former rolls. A shrink sock is most commonly a fabric similar to a plastic wire, which is heated after it has been fitted onto the roll, and the sock is tightened onto the roll face when it shrinks. The keeping of the sock in contact with the roll can be secured further, in a way in itself known, by means of collars or bands to be fitted at the ends of the suction roll. One of the advantages of this embodiment is the reliability of the solution, because the technique is in itself conventional and known from the prior art. When necessary, for example, owing to contamination, the shrink sock can be replaced readily, and, moreover, the structure of the surface fabric can be optimized readily for various purposes (for example, in order to avoid marking). The shrink sock can, of course, also be a separate fabric of some other type to be fitted onto the face of the suction roll.
The mantle of the roll can also be made of a porous material. A suction roll made of a porous material is in itself known from other, small-scale applications (see, e.g., U.S. Pat. No. 3,630,424; column 1). One of the advantages of this solution is simplicity. This application is suitable in particular for the final end of the dryer section.
The roll can also be an entirely traditional suction roll in itself known with a drilled mantle, and possibly also grooved, however, with the difference being that both the holes and the grooves, if any, are smaller than those in a traditional suction roll in a dryer section, at least in the initial part of the dryer section, in order to avoid marking. The diameter of the holes is 0.5 . . . 3 mm, most advantageously 1 . . . 2 mm. The width of a groove is 0.5 . . . 3 mm, most advantageously 1 . . . 2 mm, the width of the ridge between the grooves is 2 . . . 10 mm, most appropriately 2 . . . 5 mm, and the depth of a groove is 2 . . . 4 mm. A suction roll with small holes is also known from other applications (see U.S. Pat. No. 3,630,424). An advantage of this embodiment is a traditional technique of manufacture.
In respect of the rest of its construction, the suction roll of a dryer section in accordance with the present invention is, for example, the applicant""s a suction roll sold by the applicant under the name VAC roll (FI 83,680), or a conventional suction roll for a dryer section as described, for example, in the patent GB 2,125,461, in which suction roll there is a suction sector in the interior of the suction roll. If necessary, the suction roll is provided with a blow sector, by whose means it is guaranteed that the web is separated from the face of the suction roll. In a dryer section, such sucking and blowing rolls were used earlier as felt/wire guide rolls between the drying cylinders in twin-wire draw in order to air-condition the xe2x80x9cpocketxe2x80x9d that remains between the cylinders. The scope of the present invention also includes suction rolls in which the vacuum is produced by means of a suction box placed outside the roll (see, e.g., U.S. Pat. No. 4,359,828 and FI 943654).
The scope of the invention also includes the idea that suction rolls of different constructions/sizes are used in different parts of the dryer section. For example, in the initial part of the dryer section, it is more advantageous to use a suction roll with a shrink sock than a porous roll so as to avoid blocking of possible holes in the roll.
The scope of the invention also includes the idea that various embodiments of the present invention are applied together with traditional cylinder drying, or that different embodiments of the dryer section in accordance with the present invention are used in different parts of the dryer section.
An exemplifying embodiment of the present invention includes a suction roll and an impingement drying hood fitted on said roll, which have been fitted between two normal groups with single-wire draw. When the aim is to obtain additional drying capacity by means of the solution, one or several such units are fitted in the area of principal evaporation. From the point of view of regulation of curl, the unit is fitted preferably in the final end of the dryer section.
In a suction roll in the dryer section in accordance with the invention, there can be a xe2x80x9cdraw suctionxe2x80x9d of high vacuum in order to make the web to adhere to the roll face, after that there is favourably a less intensive xe2x80x9cholding suctionxe2x80x9d, and finally there is a xe2x80x9cblowxe2x80x9d zone that secures the transfer of the web.
In a preferred embodiment of the present invention, the invention is utilized for regulation of curl and, if desired, also for regulation of the moisture profile. In this embodiment, the last cylinder in the dryer section has been substituted for by a suction roll, in accordance with the invention, against which suction roll impingement drying is carried out. In this way drying of the web is accomplished from the opposite side, compared with the drying cylinders of single-wire draw. By means of regulation of the blow velocity, and the moisture and the velocity of the medium, an efficient mode of regulation of curl is obtained.
According to a preferred alternative embodiment of the invention, for example, a large-diameter suction roll provided with a shrink sock is placed between the press section and a cylinder dryer group, and impingement drying hoods are fitted onto said suction roll. This embodiment is particularly favourable when it is desirable to increase the dry solids content of the web before the first group of drying cylinders and in this way to improve the runnability of the dryer section. By means of this solution, an impingement drying distance of considerable length is obtained in a relatively short space in the longitudinal direction, and, in this way, an evaporation capacity higher than that of, for example, horizontal impingement drying is obtained.
In a dryer section in accordance with the above embodiment, the suction roll can also be a VAC roll with no inside suction sectors, and the web is introduced onto said suction roll by means of an upper press fabric, either a belt or a felt.
According to an embodiment of the present invention, a drying cylinder with single-wire draw is substituted for by a suction roll provided with a wire. In particular, when accomplished in the final end of the dryer section, the web can be dried in this way from its top face, and a double-sided drying favourable in view of the control of curl can be achieved.
According to an embodiment of the invention, the suction/blow roll whose face the web follows is placed between two drying cylinders so that it forms a, preferably light, nip with said cylinders. On the drying cylinders, the web is pressed by the drying wire. In this solution, blowing can be arranged from the roll into an upper pocket, in which case both ventilation of the pocket is produced and separation of the web by the effect of suction/adhering of the web to the face of the drying cylinder is secured. This solution is particularly usable when attempts are made to minimize the cross-direction shrinkage of the web.
In an embodiment of the invention, for example, a VAC roll provided with a shrink sock (fabric sleeve) is fitted between two drying cylinders in contact with said cylinders. On the cylinders, there is no wire to press the web against the cylinders. The cylinders and the hood fitted on them operate, so to say, in a way similar to a Yankee cylinder and a hood. By means of this solution, a really high drying capacity can be provided in a single-row dryer section. With this solution, the web shrinks relatively freely on the faces of the cylinders, which can be counteracted, for example, by means of suction zones fitted in the lateral areas, by means of which zones the edges of the web are held in their place. There are also paper grades in which drying-shrinkage is desirable, among other things sack papers.
In a further embodiment of the invention, the suction rolls in the upper row and in the lower row form nips with each other, and the suction rolls are preferably provided with separation blowing, which secures the runnability. It is an advantage of this embodiment, and so also of other embodiments in which a suction roll and a cylinder form a nip with each other, that in this way calendering of the web can be carried out at the same time at least partially in the dryer section.
Depending on the embodiment, by means of the present invention at least the following advantages are obtained:
increased drying capacity,
possibility to control the curl,
dryer section with no wirexe2x86x92lower costs,
pre-calendering,
easy regulation of moisture profiles by means of blow hoods,
prevention of cross-direction shrinkage, and/or
good runnability and easy removal of broke.